The sol-gel transition of aqueous alginate solutions induced by calcium cations has been studied by using dynamic light scattering.A critical relaxation was observed near the gel point

which was characterized by the power law of the time-intensity correlation function g(2)(t)-1∝t-μ.The comparison of this critical exponent μ with the critical exponent n for mechanical relaxation obtained with dynamic rheology measurements shows μ=2n for the Ca-alginate sol-gel transition.The critical exponent μ was found to be proportional to q2

q is the scattering vector

and the proportional coefficient almost equaled to the correlation length of the critical gel.